[Bug tree-optimization/71437] [7 regression] Performance regression after r235817

[amker at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71437

--- Comment #7 from amker at gcc dot gnu.org ---
I think it's not PRE's fault.  The input to PRE is already sub-optimal to be
handled.
Look at the source code:

        for( i = 0 ; i < ( I - 1 ) ; i++ )
        {
            if( ( L < pL[i+1] ) && ( L >= pL[i] ) ) 
              break ; 
        }

        if( i == ( I - 1 ) ) 
          L = pL[i] ; 
        LD = (float)( L - pL[i] ) /
                        (float)( pL[i + 1] - pL[i] ) ; 

Following path from the "break" statement, we know i < (I - 1) must be
satisfied, so control flow can go from "break" directly to assignment to LD. 
This is the case before the change.
After r235817, all paths from the loop (including the break edge) go to if (i
== ( I - 1) ) statement.  Because in if-then branch, L was assigned, and we
don't know if pL[i] is modified or not by this assignment by aliasing rule,
there is no redundancy for load of pL[i].

Thanks.